1. Field of the Invention
The invention relates generally to implantable medical devices and their methods of use for stabilizing skeletal bone, and relates more particularly to implantable medical devices and their methods of use for stabilizing the cervical vertebrae of a human spine.
2. Background of the Related Art
In humans, the spine is adapted to function as the central column of the skeleton, thereby permitting the erect posture that is characteristic of humankind. The human vertebral column includes adjacent sections of vertebrae that form curves of opposite sense in the sagittal plane. The central, or thoracic, section of the vertebral column is convex posteriorly, whereas the lumbar (lower back) and cervical (neck) sections of the vertebral column are concave posteriorly. The posteriorly concave curvature of the cervical and lumbar vertebral columns is known as lordosis.
The cervical vertebral column, with which the present invention is particularly concerned, includes seven stacked vertebrae making up two anatomically and functionally distinct segments. The superior, or suboccipital, segment contains the first vertebrae, the atlas, immediately adjacent the occiput of the skull, and a second vertebrae, known as the axis. The inferior segment contains five vertebrae that link the inferior surface of the axis to the superior surface of the first thoracic vertebrae. The atlas and the axis are each uniquely shaped, and are connected to each other and to the occiput to permit three degrees of freedom. The vertebrae of the inferior segment, in contrast, are all alike and are restricted to certain movements: flexion and extension in the sagittal plane, and lateral flexion with rotation.
In healthy individuals with normal anatomy, the vertebrae of the cervical column are held together and to the remainder of the skeleton by a complex arrangement of ligaments, tendons and muscles, resulting in a columnar frame that is significantly plastic, yet stabile and capable of rigidity. The cervical vertebrae play another very important role as a mechanical protector of the neuraxis, or spinal cord. Consequently, any loss of stability of the cervical vertebrae can present a serious threat to the integrity of the neuraxis, and thus to the life, mobility and health of the individual. Trauma or degenerative disease can result in a loss of stability of cervical vertebrae that requires surgical intervention. In some cases, implantation of stabilizing plates using bone screws to link adjacent vertebrae together may be indicated.
In the past, a posterior surgical approach was often used to implant plates or braces to stabilize the cervical spine. Beginning in the 1950's an anterior approach was developed. The anterior approach increased in popularity over the years and began to eclipse the posterior approach in the 1970's. In the anterior approach, the cervical plate spans and is affixed to adjacent cervical vertebrae on their anterior surfaces. The use of the anterior surgical approach and anterior cervical plating has certain advantages. These advantages include maintaining the patient in the supine position, requiring minimal tissue disruption for anatomic dissection, the ability to obtain anterior decompression of vertebrae, and exploiting an optimal environment for promoting bone fusion, i.e., bone under compression.
During early development of the technique of anterior cervical plating, surgeons used available bone plates and screws that had been developed for other bone stabilizing operations. W. Caspar developed the first plate and screw system specifically for use in the cervical spine. The Caspar plate could be contoured to reestablish the normal cervical lordosis. Some of the early anterior cervical plating systems involved a non-constrained arrangement in which bone screws, received through elongated holes or slots in the plate, were not rigidly affixed to the plate at their interface with the plate. This non-constrained arrangement permitted relative movement between the screw heads and the plate, which provided less stress shielding of the bone and allowed for subsidence. To prevent toggling and loosening of the bone screws, non-constrained cervical plating systems often required the bone screws to be implanted with bicortical purchase, in which the bone screw engaged the posterior cortical bone of the vertebrae in addition to passing through the anterior cortical bone.
Many surgeons were insecure with the use of bicortical purchase because of the risk of drilling through the posterior cortex and causing direct neural injury. As a consequence, constrained cervical plating systems using unicortical screw purchase gained in popularity. Concerns about screw loosening resulting in esophageal injury, as well as the need to secure the unicortical purchase screw against toggling, led to the development of mechanisms for locking the head of the screw to the plate. Such locking mechanisms prevented the screw from backing out of the plate and resisted any tendency of the screw to pivot relative to the plane of the plate. In some constrained anterior cervical plating systems, the relationship between the bone screw and the corresponding hole in the plate was such that the axis of the screw was fixed at a predetermined angle relative to the plate. To provide the surgeon with greater flexibility in placing the plate and in selecting the optimal orientation of the screw shaft, other constrained anterior cervical plating systems provided for multi-axial placement of the bone screw. Typically, in multi-axial systems, the bone screw can be placed at any angle up to about 10.degree. to 20.degree. from normal to the surface of the plate. After the plate is in place and the bone screws are installed and tightened at the selected angle, a locking mechanism is activated to maintain the screw at the selected angle.
Currently, the most desired characteristics of an anterior cervical plating system include the ability to use unicortical screw purchase, with the option to employ bicortical purchase, variability of the axis of screw placement in the superior, inferior, medial and lateral directions together with fixation of the screw to the plate, and titanium construction. Titanium is preferred because of its minimal interference with the magnetic resonant imaging (MRI) technique used for postoperative evaluation. Bendability or precurvature of the plate is also desired to accommodate or restore the natural lordosis of the cervical spine. These, and other desirable features and advantages, are provided by the present invention, particular embodiments of which are described below.